Semiconductor processing often utilizes plasma processing to etch, clean or deposit material on semiconductor wafers. All such processes are advantageously highly uniform over the entire surface of a processed wafer. Wafer sizes have increased while feature sizes have decreased, significantly over the years, so that more integrated circuits can be harvested per wafer processed. Typical wafer diameters increased from about 2 or 3 inches in the 1970s to 12 inches or more in the 2010s. Over the same time frame, typical minimum feature sizes of commercial integrated circuits decreased from about 5 microns to about 0.015 microns. Processing smaller features while wafers grow larger requires significant improvements in processing uniformity. Plasma processing of workpieces other than wafers may also benefit from improved processing uniformity.